The present invention relates to a receiver receiving a multi-carrier digital broadcast signal such as an orthogonal frequency division multiplexed (OFDM) signal, more particularly to the method of symbol synchronization employed in the receiver.
OFDM broadcasting enables digital data to be transmitted to mobile receivers despite such problems as multipath reception and fading. OFDM systems are already being deployed, a prominent example being the digital audio broadcast (DAB) system standardized in Recommendation BS.774 of the Radiotelecommunication Sector of the International Telecommunications Union (ITU-R), entitled xe2x80x9cService requirements for digital sound broadcasting to vehicular, portable, and fixed receivers using terrestrial transmitters in the VHF/UHF bands.xe2x80x9d
A DAB signal is divided into frames, each comprising a pair of synchronization signals followed by a number of data symbols with respective guard intervals. The first synchronization signal is a null symbol with zero amplitude. The second synchronization signal is a phase reference symbol in which known data are transmitted.
A DAB receiver carries out frame synchronization and approximate symbol synchronization by using an envelope detector to detect the null symbol. A conventional DAB receiver refines symbol synchronization by canceling the known data from the demodulated phase reference symbol, converting the results to the time domain to obtain a channel impulse response waveform, and detecting peak values in the channel impulse response waveform. When multiple peaks are present, the conventional receiver selects one of the peak values and synchronizes symbol demodulation according to the selected peak.
Reception conditions can change rapidly, especially in a mobile environment. New signal components may appear abruptly, as when a mobile receiver emerges from an area shielded by tall buildings or topographic obstructions and starts receiving a strong signal from a nearby transmitter. It is therefore advisable for the conventional receiver to leave a timing margin to allow for the sudden appearance of new peaks in the channel impulse response waveform. A conventional receiver that does not allow such a timing margin will experience frequent inter-symbol interference, with adverse effects on receiver performance.
The provision of a timing margin, however, has the effect of narrowing the guard interval around each symbol. The narrowed guard interval may be unable to accommodate large delays between different signal components, again leading to inter-symbol interference. When the large delays are due to persistent conditions, such as the arrival of signals from different transmitters in a single-frequency network, the performance degradation is persistent.
The conventional digital broadcast receiver also controls the sampling frequency on the basis of the selected peak in the channel impulse response waveform. This leads to the problem of unstable frequency control under multipath reception conditions, when the selection shifts frequently from one peak to another.
Further details will be given in the detailed description of the invention.
An object of the present invention is to provide a digital broadcast receiver that extracts appropriate timing control information from a channel impulse response waveform under adverse reception conditions, including multipath reception and the large delays encountered in single-frequency networks.
The invented digital broadcast receiver receives a multi-carrier signal that periodically includes a synchronization signal. The receiver has a data demodulator that demodulates the multi-carrier signal by use of a sampling window to obtain demodulated data, and a control unit that controls the timing of the sampling window. The control unit comprises a waveform generator that generates a channel impulse response waveform from the demodulated data of the synchronization signal, and a multiply-add unit. The multiply-add unit multiplies values in the channel impulse response waveform by corresponding weighting coefficients in a predetermined weighting function, thereby obtaining product values, and adds the product values to obtain a weighted sum. The control unit adjusts the timing of the sampling window according to the weighted sum.
The control unit also adjusts the frequency of a voltage-controlled oscillator that determines the sampling rate of an analog-to-digital converter supplying sample values to the data demodulator. In one aspect of the invention, the control unit performs this adjustment according to the difference between the timing of a maximum peak value in the channel impulse response waveform and an adjusted peak timing. The adjusted peak timing is the timing of the maximum peak value in the channel impulse response waveform obtained from a preceding synchronization signal, adjusted to compensate for the amount by which the sampling window timing was adjusted when the preceding synchronization signal was processed.
By controlling the sampling window timing according to a weighted sum of all values in the channel impulse response waveform, the invented digital broadcast receiver is able to minimize inter-symbol interference despite the presence of widely separated signal components, while leaving as large a margin as possible to accommodate new signal components.
By controlling the sampling frequency according to the maximum peak timing, the invented digital broadcast receiver avoids unstable frequency control due to the frequent selection of different peaks.